1. Field of the Invention
The present invention relates to a method for controlling an array antenna apparatus, capable of changing a directive characteristic of the array antenna apparatus including a plurality of antenna elements. In particular, the present invention relates to a method for controlling an array antenna apparatus, capable of adaptively changing a directivity characteristic of an electronically controlled radiator array antenna apparatus (Electronically Steerable Passive Array Radiator (ESPAR) Antenna; hereinafter referred to as an ESPAR antenna). Further, the present invention relates to a method for calculating a signal to noise ratio of a radio receiver for calculating the signal to noise ratio of a received signal received by the radio receiver, and also, to a method for adaptively controlling a radio receiver utilizing the method for calculating the same.
2. Description of the Prior Art
An ESPAR antenna of prior art is proposed in, for example, a first prior art document of “T. OHIRA et al., “Electronically steerable passive array radiator antennas for low-cost analog adaptive beamforming”, 2000 IEEE International Conference on Phased Array System & Technology pp. 101-104, Dana point, Calif., May 21-25, 2000”, and Japanese Patent Laid-Open Publication No. 2001-24431. This ESPAR antenna is provided with an array antenna including a radiating element fed with a radio signal, at least one parasitic element that is provided apart from this radiating element by a predetermined interval and is fed with no radio signal, and a variable reactance element connected to this parasitic element. Further, this ESPAR antenna can change a directivity characteristic of the array antenna by changing the reactance value of the variable reactance element.
As a method for adaptively controlling this ESPAR antenna on the reception side, the following method is generally used. That is, a learning sequence signal is preparatorily included in the head portion of each radio packet data on the transmission side, and the same signal as the learning sequence signal is generated also on the reception side. On the reception side, the reactance value of the variable reactance element is changed to change its directivity characteristic on such a criterion (estimation criterion) that a cross correlation between the received learning sequence signal and the generated learning sequence signal becomes the maximum. By this operation, the directivity of the ESPAR antenna is made to have an optimum pattern, i.e., such a pattern that a main beam is directed in the direction of a desired wave, and nulls are formed in the directions of interference waves.
As a method for adaptively controlling the above-mentioned ESPAR antenna on the reception side, it is widely performed to adaptively control an array antenna by a method of, for example, the constant modulus algorithm for performing adaptive control so that the amplitude of the received radio signal becomes constant when the transmitted radio signal is modulated by a modulation method of a constant amplitude such as frequency modulation. However, there has been such a problem that the method has not been able to be used when the transmitted radio signal is modulated by a modulation method that includes amplitude modulation.
However, the above-mentioned prior art example needs a reference signal such as a learning sequence signal, and is required to make the reference signals coincide with each other on both the transmission side and the reception side, and this leads to such a problem that the circuit for adaptive control has been complicated.
Moreover, in order to adaptively control a signal equalizer and a signal filter in the radio receiver, it is required to estimate and calculate a signal to noise power ratio. However, it has been unable to calculate the ratio in real time for the received signal.